Couscous is not actually a grain but is a mix of two types of flour, so should really be termed pasta and best though of this from a cooking angle. Couscous comprises semolina and fine white flour, which is rehydrated to become sticky; each price of semolina is rolled in the sticky white flour to cover them in a film of flour paste.
These couscous pieces are steamed, not boiled, then finished over Moroccan stews, heavily spices with ginger, cumin and coriander seeds, plus perhaps saffron. Couscous is sometimes made with other grains such as bulghur but shouldn't really be.
A variant on couscous is Mougrabieh which is massive 5 mm pearls that hail from the Lebanon. Mougrabieh is made from finely ground semolina or durum wheat flour which has been made into balls with a little water.